Ingredients:

  • 4 boneless, skinless chicken breasts, about 6 oz (170 g) each — patted dry, trimmed
  • 1 teaspoon kosher salt (or 3/4 tsp table salt)
  • 1/2 teaspoon black pepper
  • 1 tablespoon olive oil (15 ml) for searing
  • 3 large yellow onions (about 24 oz / 680 g total), peeled and thinly sliced
  • 2 tablespoons unsalted butter (28 g)
  • 1 tablespoon olive oil (15 ml) — helps prevent burning
  • 1 teaspoon granulated sugar (optional — speeds caramelization)
  • 1/2 teaspoon kosher salt (or 1/4 tsp table salt), plus more to taste
  • 1/2 cup dry white wine (120 ml) or dry sherry (optional; use extra beef broth if omitted)
  • 2 cups low-sodium beef broth (480 ml) — or 1 3/4 cups chicken broth + 1/4 cup beef broth for milder flavor
  • 1 teaspoon Worcestershire sauce (5 ml)
  • 1 teaspoon Dijon mustard (5 ml)
  • 1 tablespoon all-purpose flour (8 g) — optional, to slightly thicken the sauce
  • 1 cup grated Gruyère cheese (about 100 g) — can mix with Swiss if needed
  • 2 tablespoons chopped fresh parsley (optional, for garnish)
  • Optional for serving: crusty bread or toasted baguette slices, mashed potatoes, rice or roasted vegetables

Instructions:

  1. Prepare: Preheat broiler (or oven to 400°F / 200°C if you prefer baking finish). Slice onions thinly; pat chicken dry and season with salt and pepper.
  2. Caramelize the onions: Heat a 10–12 inch ovenproof skillet over medium, add butter and 1 tablespoon olive oil. Add sliced onions, sprinkle with 1/2 teaspoon salt and optional 1 teaspoon sugar. Cook low-and-slow, stirring occasionally and lowering heat as needed, until onions are deep golden-brown and jammy, about 30–40 minutes.
  3. If using, stir in 1 tablespoon flour in the last 1–2 minutes of caramelizing to remove raw taste and help thicken the sauce.
  4. Deglaze and build the sauce: Increase heat to medium-high, add 1/2 cup dry white wine (or a splash of broth) to deglaze, scraping up browned bits. Add 2 cups low-sodium beef broth, 1 teaspoon Worcestershire sauce and 1 teaspoon Dijon mustard. Bring to a simmer and reduce 5–10 minutes until slightly thickened; adjust seasoning to taste.
  5. Sear the chicken: In a separate pan (or clear space in the same skillet if preferred), heat 1 tablespoon olive oil over medium-high. Sear chicken 2–3 minutes per side until golden brown but not cooked through. If using the same skillet, transfer onions to a bowl before searing and return them afterward.
  6. Combine and braise: Nestle seared chicken into the onion sauce so onions cover the breasts. Spoon sauce over the chicken and simmer gently 8–12 minutes until chicken approaches doneness (internal temp ~155–160°F / 68–71°C). The final broil will finish cooking to a safe 165°F (74°C) after resting.
  7. Finish with cheese under the broiler: Preheat broiler high. Sprinkle 1 cup grated Gruyère evenly over each breast and the surrounding onions. Broil 2–5 minutes, watching closely, until cheese is melted, bubbly and lightly browned.
  8. Rest and serve: Remove from oven and let rest 3–5 minutes to reach final temperature. Garnish with chopped parsley and serve with crusty bread, mashed potatoes, rice or roasted vegetables as desired.